First, the extent of variable region diversity among murine antibodies to the phosphorylcholine antigen will be examined. Homogeneous antibodies have been produced by hybrid cells from the fusion of normal antibody-secreting cells and myeloma cells. The amino acid sequence of the variable regions of the heavy and light chains will furnish information concerning the nature of antibody diversity. Second, the heavy chain class of immunoglobulin on the membrane of B cell progenitors will be compared to the class of antibody that is secreted by their clonal progeny. B cells will be stained with fluorescent antisera and separated according to membrane immunoglobulin class using a fluorescence-activated cell sorter. Separated cells will be individually stimulated with antigen in vitro, and the heavy chain class of the secreted antibody made by clonal progeny will be determined by radioimmunoassay. This study may provide insights into the relationship between membrane components of precursor cells and cytoplasmic molecules in differentiated cells.